Modern Methods in Heterogeneous Catalysis Research
Adsorption and desorption Wolfgang Ranke Department of Inorganic Chemistry Fritz-Haber-Institut der MPG Faradayweg 4-6, DE 14195 Berlin For script: see homepage or mail to: [email protected] Literature: R.I. Masel, Principles of adsorption and reaction on solid surfaces, Wiley, New York(1996). K. Christmann, Surface physical chemistry, Steinkopff, Darmstadt (1991). G.A. Somorjai, Introduction to surface chemistry and catalysis, Wiley, New York (1994). M. Henzler, W. Göpel, Oberflächenphysik des Festkörpers, Teubner, Stuttgart (1991). W. Ranke and Y. Joseph, Determination of adsorption energies and kinetic parameters by isosteric methods, Phys. Chem. Chem. Phys. 4 (2002) 2483.
FHI-Berlin, 28.11.2008 W. Ranke, Dept. AC, Fritz Haber Institute of the MPG, Faradayweg 4-6, 14195 Berlin, Germany Adsorption and catalysis Contents
Part I 1. Some definitions and concepts 2. Physisorption and chemisorption 3. Energetics: Potential curves
Part II 4. Adsorption-desorption-equilibrium: Isotherms and isobars 5. A-D energetics: Isosteric heat of adsorption 6. A-D kinetics: The shape of isotherms/isobars 7. The magnitude of ν: Entropy of the activated state 8. Further isotherms for mono- and multilayer adsorption 1. Some definitions and concepts Note: Usually we talk about adsorption. But: desorption equally important. desorption = inverse adsorption
The sticking process. Example: Dissociative
adsorption of H2O on Si(001). The Si(001) surface relaxes in order to minimize the number of unsaturated (“dangling”) bonds by formation of asymmetric dimers. Relaxation is changed upon adsorption.
(Review on water: M.A. Henderson, Surf. Sci. Reports 46 (2002) 1; calculations: A. Vittadini et al. Phys. Rev. B 52 (1995) 5885)
Adsorption: molecules from gas phase or solution bind in a layer of condensed phase on a solid or liquid surface. The molecules are called adsorbate, the substrate is called adsorbent. The process of binding is called adsorption. Removal of the molecules is called desorption. Accomodation and Sticking: The incoming particle has to get rid of its translational and rotational energy and has to assume a suitable spatial configuration. The surface may have to rearrange: Accomodation. If this does not happen: Reflection. Sticking: The sticking coefficient s or sticking probability is the ratio of particles being bound compared to those hitting the surface. The value of s often reaches 1 but may also be almost infinitely small.
2 -2 -1 Rate of molecules striking 1 cm of N2 at 273 K: jN (cm s ) Time needed surface: for 1 ML (s=1) 1000 mbar 3×1023 5×10-8 s 22 1/2 -2 -1 20 -6 jN = 2.63×10 p/(MT) cm s , 1 mbar 3×10 5×10 s p in mbar, M in g/mol. 10-3 mbar 3×1017 5×10-3 s 10-6 mbar 3×1014 5s For comparison: 10-10 mbar 3×1010 5×103 s Density of atoms on solid surfaces Pt(111): 1.5×1015 cm-2 Si (001): 6.8×1014 cm-2 Monolayer and multilayer adsorption
Multilayer incommensurate
SK-growth: 1st layer commensurate, further layers incommensurate
Multilayer commensurate
γs free energy of bare substrate; Free surface energies If γin is negative: γf,n free energy of adlayer-film of thickness n; (= surface tensions) dissolution.
γin free energy of interface; are always positive: commensurate growth for first layer assumed; making a surface misfit causes strain costs always energy. Not treated here:
Formation of new bulk components or bulk phases as result of reactions like oxidation alloying dissolution etc.
Adsorption from liquid phase 2. Physisorption and chemisorption 2.1 Physisorption
Nonpolar gases on nonpolar surface, no chemical interaction: Van-der-Waals (induced-dipole – induced-dipole) interaction. Starting from the Lennard-Jones 6-12 potential (interaction between single atoms or molecules), 12 6 V(R)=4ε [(σ/R) –(σ/R) ], ε depth of potential minimum; σ=0.891 Re (equilibrium distance). the London-equation, ρs: density of surface atoms; s 6 C : London constant, depends on ⎛ r )(2.0 1 ⎞ Lond zE )( = πρC ⎜ e − ⎟ polarizabilities and ionization potentials; sMMs Lond 9 3 s ⎜ ⎟ re : equ. distance when the adatom interacts ⎝ M zz M ⎠ with one atom only;
zM: distance of adatom from surface. was derived for non-polar gases on non-polar non-conducting surfaces. on conducting surface: Same functional dependence. Compare: Characteristic: Thermal E (300 K) zM >2Å; RT~2.5 kJ/mol ΔHad: 1.4 kJ/mol (He/graphite) – 42 kJ/mol (C5 H12 /graphite).
Even if ΔHad is small, Θ may be high at high pressure, see isotherms/isobars Van-der-Waals interaction is responsible for condensation of nonpolar gases like inert gases, alkanes, aromatics... 2.2 Chemisorption
Electronic structure, bond lengths and bond angles strongly affected. Substrate and adsorbate not separable. Simple models do not exist. Theoretically, the whole system (substrate + adsorbate) has to be treated quantum mechanically which has become feasible during the last years.
Characteristic:
zM = 1 – 1.5 Å; ΔHad up to several hundred kJ/mol.
Covalent - (Example: H2 O/Si(001)., see above. Ionic - (Example: Alkali adsorption)
Acid-base - (Example: H2 O/Fe3 O4 (111)) H-bond - (Example: H2 O-dimers and islands, lateral interaction) Model for the interaction and dissociation of water on an acidic Fe-site on the
surface of Fe3O4(111); the Fe is formally Fe3+ (Y. Joseph et al. Chem. Phys. Lett. 314 (1999) 195).
Hardness (Pearson) η: Measure of the capability to share electrons. Hard (η large): Species that form strong ionic bonds; Medium: Species that form mixed covalent and ionic bonds; Soft (η small): Species that form strong covalent bonds;
Charge acceptor: acid; charge donor: base. Strong interaction between a hard acid and a hard base or a soft acid and a soft base. Weak interaction between a hard acid and a soft base.
+ + 2+ 3+ 3+ 3+ 3+ Hard acids H , Li , Mg , Cr , Co , Fe , Al , Al(CH3 )3 , bulk Si Borderline acids Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Ru2+, Os2+, Ir3+, Rh3+ + + 2+ 2+ Soft acids Cu , Ag , Pd , Pt , Ga(CH3 )3 , O, Cl, N, RO, ROO, metallic atoms, metal clusters - - Soft bases I , CO, C2 H4 , P(C6 H5 )3 , C6 H6 , H , H2 S, metal surfaces - 2- Borderline bases C5 H5N, NO2 , SO3 , C6H 5 NH2 ------Hard bases F , Cl , H2 O, NH3 , OH , CH3 COO , RO , ROH, O
(Masel Table 3.8, p. 145) 3. Energetics: Potential curves
R.J. Hamers, Surf. Sci. 583 (2005) 1. „Ten or twenty phonons“transfer A number of phonons is recently their energy collectively to an sequentially absorbed. The normal observed electron which is excited into adsorbate-substrate vibration case an antibinding state from which gains amplitude until dissociation occurs. dissociation occurs.
Classical Interference-generated freak wave Resonance-like amplitude build-up analog: (Monsterwelle, Kaventsmann) (Resonanzkatastrophe)
Adsorption, accomodation: invert all arrows Potential curves for approach of a non-dissociated particle to a surface: physisorption, chemisorption and sum curve (dotted).
ΔHphys << ΔHchem typically ΔHphys =20 – 80 kJ/mol, ΔHchem > 100 kJ/mol.
But: Magnitude of ΔHad alone is not a sufficient criterion for the distinction: Change of geom. and electron. structure possible.
Potential curve for the dissociative adsorption of a
B2 molecule on a surface (Masel fig. 3.8, p.119.)
Potential curves for (a) pure molecular adsorption, (b) activated dissociative adsorption (c) nonactivated dissociative adsorption (Masel fig. 3.9, p.119)
3.2 Dissociative adsorption, several sequential steps
The mechanism of ethylene decomposition on Pt(111). (Proposed by Kesmodel et al. [1979] and confirmed by Ibach and Lehwald [1979], Masel fig. 3.10, p.121).
Example for “irreversible” adsorption. Irreversible is not strictly the correct term: It is irreversible under usual experimental conditions where a part of the formed species
(here hydrogen in the form of gaseous H2 ) is irreversibly removed from the system. 3.3 Adsorption site Surfaces are inhomogeneous interaction with certain sites is stronger than with others. Intrinsic inhomogeneity from periodic atomic structure ordered adsorbate structures
Different adsorption sites on a close-packed surface: Linear or on-top site, bridgebound site and triply coordinated or threefold hollow site (Masel fig. 3.13, p.123 ).
Example: CO/Pt(111)
Binding sites for CO on Pt(111): Two ordered adsorption arrangements assumed sequentially with increasing coverage:
Θr = 0.33 and 0.5 (Masel fig. 3.14, p.123).
Example: H2 O/Fe3 O4 (111),
Clean and water covered surfaces of FeO(111) and Fe3O4(111). Coverages and kind of adsorbed species deduced from UPS measurements. FeO(111) is O-terminated. The interaction is weak but still determined by
the position of Fe in the second layer. Fe3O4 is terminated by ¼ ML of Fe. Water dissociates. The coverage is compatible with OH being adsorbed on the Fe sites and H binding to O-sites. Site (1) in (d) is the proposed transition state. (Y. Joseph et al. J. Phys. Chem. B 104 (2002) 3224;). Further water bridges OH and H via H-bonds.
A H3O-OH like “dimer” is formed. ( M.E. Grillo et al., Phys. Rev. B 77 (2008) 075407)
Extrinsic inhomogeneity from defects (steps, domain boundaries, kinks, adatoms, vacancies, contaminants…)
Model of a heterogeneous surface with different surface sites. They are distinguished by their number of nearest neighbours. (Somorjai, fig. 2.6, p. 41.)
ΔHad = f(substrate, Θ).
• material dependence • site dependence • ads.-ads.-interaction: repulsive or attractive • ads.-subs.-interaction may induce structural changes in substrate (reconstruction, relaxation) Remember these terms:
adsorption, desorption, adsorbent, adsorbate, substrate, accomodation, sticking, sticking coefficient, monolayer, multilayer;
physisorption, chemisorption, hardness, hard/soft acid/base;
potential curve;
adsorption: molecular, dissociative, non-activated, activated reversible and irreversible;
adsorption sites: linear, on-top, bridge, triply coordinated, threefold hollow site; 4. Adsorption-Desorption-Equilibrium: Isotherms and isobars Adsorption rate:
(s0 : initial sticking coefficient, n : reaction order for adsorption ; ra = ra(p, s0, n, T, Ea, Θ). Ea: activation energy for adsorption, Θ : coverage)
Desorption rate: (ν : frequency- or prefactor for desorption; n: reaction order for r = r (ν, n, T, E , Θ), d d d desorption; Ed: activation energy for desorption) increases with Θ.
After a while, when adsorption-desorption equilibrium condition ra = - rd , an equilibrium coverage Θeq will establish.
Θeq(T)p T variable, p constant: Isobar Θeq(p)T p variable, T constant: Isotherm
How to determine Θ ? High surface area (powders, pellets…): pressure decrease by adsorption (e.g. BET), TDS. Low surface area (single crystals): Δφ, LEED, IRAS, UPS, TDS.
The difference and Ed -Ea determines the adsorption energetics
p, s0, n and ν determine the kinetics.
Aim: measure Θ(p,T), deduce energetic ΔH(Θ ) = q and kinetic parameters from it. st qst: isosteric heat of adsorption
The fife types of adsorption isotherms described by Brunauer[1945]
(Masel fig. 4.4, p.238).
Type Where to be found I monolayer adsorption II multilayer adsorption, BET isotherm III island or droplet nucleation necessary for adsorption IV pore filling, followed by outer-surface adsorption V pore filling with nucleation (like III), followed by outer surface adsorption 5. A-D energetics: Isosteric heat of adsorption Measure a set of isotherms or isobars,
deduce p-T-pairs for Θeq=const, plot ln(p) vs(1/T), use Clausius-Clapeyron equation Strictly, CC is only applicable if adsorbate is separable from substrate (no strong interaction, physisorption). d(ln p)/d(1/T) = - ΔH/R or Kinetic considerations: Applicable if r=C exp(-E /RT) ln p = - ΔH/RT + const. act (Arrhenius law). (W. Weiss, W. Ranke, Prog. Surf. Sci. 70 (2002) 1). or, since ΔH may be Θ-dependent,
ΔH(Θ) = qst = isosteric heat of adsorption; isosteric means: for constant coverage.
Isosteric methods
Advantage: CC is deduced from equilibrium thermodynamics and independent of kinetics. Disadvantage: adsorption must be reversible. (If not: => calorimetric methods). adsorption and desorption must be sufficiently fast (equilibrium established within reasonable time) . Examples: 1. Adsorption on a metal: CO/Pd(100)
high low pressure pressure Binding energy Binding
high coverage
14 2 low coverage (10 molec./cm ) coverage
Coverage dependence of qst deduced from the slopes of the curves in (b). (J.C. Tracy, P.W. Palmberg, Surf. Sci. 14 (1969) 274). a) Isobars for CO adsorption on Pd(111). As a measure for the coverage, the work function change Δφ was used. In special cases, Δφ is proportional to Θ. b) Arrhenius-plots ln p vs. 1/T for different coverages (in terms of Δφ).
2. Adsorption on a semiconductor: NH3 /Ge (W. Ranke, Surf. Sci. 342 (1995) 281; W. Ranke, J. Platen, Phys. Rev. B 54 (1996) 2873)
3. Adsorption on an ionic material: Ethylbenzene/FeO(111), Fe3 O4 (111) (W. Weiss, W. Ranke, Progr. Surf. Sci. 70(2002) 1, Figs. 54, 56, 57, 58b) Adsorption of ethylbenzene (EB) is studied in connection with the investigation of its catalytic dehydrogenation to styrene.
Photoelectron Adsorption spectra of isobars clean deduced from UPS Fe3O4(111) and after measurements equilibrium of EB on coverage by FeO(111) EB at and Fe3O4(111) -9 for three values pEB=4x10
mbar and of pEB.d/le is different T. proportional to the coverage. Coverage dependence of q For different coverages (in terms of d/le), st p-T- values are taken from the isobars. deduced from the CC-analysis ln(p) vs. 1/T yields mostly straight lines. of isobars.
Their slope yields qst.
==> TDS (TPD), a transient method, evaluation depends on kinetics. 6. A-D kinetics: The shape of isotherms/isobars Kinetics: rate of adsorption and desorption dΘσ p r rA == exp − /kT)E(s Θ )f( = A Θ )f( a dt 2π mkT 0 a r r
dΘσ rA n n n r −== σν exp − /kT)E( Θ −= BΘr d dt An d r
Ea, Ed: activation energies for adsorption and desorption; s0: initial sticking probability: σA: density of adsorption sites;
Θr =Θ /Θsat: relative coverage (0<Θr<1);
νn: the frequency factor for reaction order n; n: reaction order.
I follow the convention that rd is negative since dΘ/dt is negative for desorption.
f(Θr) describes the dependence of the sticking probability s on the coverage, s = s0 f(Θr).
Langmuir: Sticking only if unoccupied site is hit. st 1 order (molecular): f(Θr) = 1-Θr . nd 2 2 order (dissociative): f(Θr) = (1-Θr) . Mobile precursor kinetics: (G) may hit an empty site: intrinsic precursor state (IP) which may be mobile. It may transform into the adsorbate state (A)
(probability Pa) or desorb (probability pd). (G) may hit an occupied site: extrinsic precursor (EP) which may migrate to an empty site and transform into an (IP) and adsorb (see above) or desorb directly
(probability pd’ ). Kisliuk: sticking behavior is determined K which depends on these probabilities only (P. Kisliuk, J. Phys, Chem. Solids, 3 (1957) 95).
If pd’ is small (stable (EP)), the sticking coefficient may be quite high almost until saturation (Θ =1). r 1− Θ 1st order: Θ )f( = r with K>0. r Θ −+ 11 )(K K=1 corresponds to Langmuirr case. (W. Ranke, Y. Joseph, Phys. Chem. Chem. Phys. 4 (2002) 2483) Check of kinetics: determine s(Θ ). Example: H2 O/Si(001):
UP-spectrum (hν=21.2 eV) Adsorption curves. Intensity The first derivative of the of Si(001), clean and after increase of the –6.2 eV peak adsorption curves represents saturation with H2O which of OH with exposure time the sticking coefficient s(Θ). -9 adsorbs as OH+H. Equal for pH2O=2x10 mbar for the The wiggles originate from species and saturation for same values of T. pressure adjustments in the 183≤T≤391 K. At 137 K, adsorption curves. condensation of H2O occurs, (W. Ranke, Surf. Sci. 369 (1996) 137) at 412 K, saturation is not yet reached. In adsorption-desorption equilibrium we have ra + rd = 0. n We had ra = A(p,T) f(Θr) , rd = -B(p,T) Θr .
Inserting and resolving for Θr yields the equations for isotherms/isobars: p(T)b Langmuir 1st order: Θ (T,p) = 1 with n = 1, f(Θ ) = 1-Θ , r r r 1+ 1 p(T)b
nd 2 p(T)b 2 Langmuir 2 order: Θr(T,p) = with n = 2, f(Θr) =(1-Θr) . 1+ 2 p(T)b Holds if the adsorption sites for the dissociation products are equivalent and for inequivalent sites as long as their site concentrations are equal. Langmuir 2nd order adsorption, quasi-1st order desorption (dissociated species immobile): + +− )(41)(21 pTbpTb 1 1 with n = 1, f( ) =(1- )2 . r pT ),( =Θ Θr Θr 1 )(2 pTb
Kisliuk 1st order:
2 with n = 1, 1 −−+ 1 + 1 )(4])(1[)(1 pTbKpTbpTb r pT ),( =Θ 1− Θr − K)1(2 Θr )f( = Θr −+ 11 )(K kTqs )/exp( kTqs )/exp( Tb )( = 0 st Tb )( = 0 st 1 2 2 qst = Ed -Ea 1 A 2πσνmkT 2 A 2πσνmkT
What should these isobars look like?
Assumption: Langmuir 1st order (n = 1), ν = 1015 s-1, 1 q = 58 kJ/mol st (both independent of Θ, data for ethylbenzene physisorbed on FeO(111)). Pressures as indicated 13 -1 a) comparison: ν1 varied (→ 10 s ); b) comparison: ν → 1013 s-1, 1 q adjusted to yield best-possible st agreement; c) comparison: Kisliuk kinetics (mobile precursor), K varied; d) comparison: Langmuir 2nd order.
(W. Ranke, Y. Joseph, Phys. Chem. Chem. Phys. 4 (2002) 2483). Example: Ethylbenzene / FeO(111), Fe3O 4(111)
Same measurement as shown above, lines: fitted isobars.
FeO(111),
β-range: qst = 58 kJ/mol, 14 -1 ν1 = 4.75x10 s . Dotted: Langmuir 1st order gives too
smooth curves.
Lines: Kisliuk 1st order, K = 0.2.
Fe3O4(111), γ-range: qst = 94…74 kJ/mol, 12 10 -1 ν1 = 5x10 …2x10 s (Θ-dependent); Langmuir or Kisliuk possible.
β-range: Fit impossible reason: phase transitions in adlayer.
(W. Ranke, Y. Joseph, Phys. Chem. Chem. Phys. 4 (2002) 2483). What should these isobars look like?
Isobars for EB/FeO(111) for high pressure conditions, extrapolated from data measured at low p, T:
Although ΔHa=qst is small, Θ gets quite high if only p is high enough.
7. The magnitude of ν: Entropy of the activated state
The transition state theory starts from the assumption that the molecule is in thermodynamic equilibrium with its environment, even during the desorption reaction.
νdes ~ probability to realize the transition state dominating: Boltzmann factor
exp(-Ed/kT); determining: configuration (here restricted direction, rotation).
probability to realize a state: partition function kT q# kT ⎛ ΔS # ⎞ partition functions It can be shown that ν ≈≈ exp⎜ ⎟ q# in the transition state st des ⎜ ⎟ q in the adsorbed state (for 1 order desorption) h qad h ⎝ R ⎠ ad
(see e.g. K. Christmann, Surface physical chemistry, Steinkopff, Darmstadt (1991), p.27ff).
# 13 -1 If q =qad , νdes ≈10 s (T=300K). If the adsorbed state is immobile and the transition state is mobile and may even rotate,
# q and consequently νdes may get much larger. If the transition state requires a complicated or “demanding” configuration which has a
# low probability of realization q and νdes may get much smaller. The agreement between measured and calculated values of νdes is poor. Nevertheless,
transition state theory gives an idea why νdes values vary so strongly.
10 -1 19 -1 Experimentally, νdes values between about 10 s and 10 s have been observed while values around 1013 s-1 do not appear to be especially probable.
Excursion: Partition function, probability and entropy
Partition function of a system: q = Σi exp(-εi / kT) = sum over all possible states, weighted by exp(-εi / kT), the prob. of occupation (Boltzmann) or number of thermally accessible states at given T; or probability to „materialize“ in the suggested arrangement or configuration (→ entropy, S=k lnW). Quantum mechanically: Discrete states, different contribution to q at 300 K: Translation: densely spaced, Δε ≈ 10-16 kT dominating in gas phase, zero in immob. ads. Adsorbed state: Rotation: less densely spaced, Δε ≈ 0.1 kT small in gas, usually zero in ads. q much smaller than in gas Vibration: not densely spaced, Δε ≈ 10 kT very small in gas, dominating in ads. → compensation by
Electronic: separated Δε ≈ 40 kT virtually zero E necessary ad
8. Further isotherms for monolayer and multilayer adsorption 8.1 Other isotherms / isobars for monolayer adsorption:
Isotherm Advantages Disadvantages Langmuir Best one-parameter isotherm Ignores adsorbate-adsorbate interactions Freundlich, Toth Two parameters No physical basis for equation Multisite Many parameters Good for inhomogeneous surfaces. Wrong physics for single crystals Tempkin Fowler Account for adsorbate-adsorbate Does not consider how the Slygin-Frumkin interactions in an average sense adsorbate layer is arranged Lattice gas Complete description of Requires a computer to calculate adsorbate-adsorbate interactions isotherm for commensurate layers Assumes commensurate Predicts arrangement of adsorption adsorbed layer Parameters used in the model are difficult to determine
(R.I. Masel, Principles of adsorption and reaction on solid surfaces, Wiley, New York (1996), table 4.1, p.246). 8.2 Multilayer adsorption: The BET isotherm (Brunauer-Emmett-Teller)
Multilayer adsorption with different ΔG , ΔG and 1 m ΔG1, ΔG2, ΔGm, respectively.
Either:
Consider each step in the isotherm separately with its own n, ν, s0, qst, add isotherms (e.g. Langmuir) for 1st, 2nd … layer. Problem: Multilayer condensation. Or: Develop an isotherm for multilayer adsorption: Brunauer-Emmett-Teller, BET-isotherm. V xc = BB VM B B −+− xcx B ])1(1[)1(
st (V : total volume of adsorbed gas; VM : volume gas in 1 layer (monolayer saturation)); xB = p/psat (p the gas pressure and psat the saturation vapor pressure of the gas at the temperature of the measurement). st cB = K1 /Km ratio of equilibrium constants for 1 layer and multilayer adsorption:
K1 −Δ 1 RTG )/exp( cB == (exp[ 1 Δ−Δ−= m RTGG ]/) Km Δ− m RTG )/exp( A plot of adsorption isotherms predicted by the
BET equation for various values of cB. (Masel fig. 4.30, p.302)
In fact, the BET isotherm does in general not very well fit measured isotherms. However, in the environment of ML saturation it usually does. If, however, ML.saturation can be unambiguously be identified in the isotherm by step structure, the ML capacity can directly be taken from the isotherm. For script: see homepage or mail to: [email protected]
Remember these terms:
adsorption-desorption equilibrium, isotherm, isobar, isosteric heat of adsorption; rates of adsorption and desorption, order of adsorption and desorption., frequency- or pre-factor; Langmuir adsorption, mobile precursor; transition state; BET isotherm.